I have to say that I am aware of the issues with Buna-N and nitrile o-rings and their expansion when exposed to CO2 but I have been using a Buna-N o-ring as the end cap seal for a few days now with no expansion issues at all. I’ve probably gone through four or five CO2 capsules. I never leave full or partially full CO2 capsules in my guns so they only get used fo a short period before the cap is removed and a new cartridge is fitted. So I think it is entirely possible to use “standard” o-rings on the end cap.
It could be either. I have further researched both the pneumatic and the CO2 variant and the pneumatic is also seen with the pellet loading shelf in front of the breech. In this example, it is possible that the pistol is cocked and only the rod is visible. Otherwise I would expect to see the knob at the end.
All the best,
Jimmie